Recently, an increasing amount of research has been focused on micro-sizing of systems for a chemical reaction and separation called a micro reactor and a micro total analysis system (μTAS) where a micromachining technology is utilized. These systems are expected to be applied to the analysis and synthesis of nucleic acids, proteins, sugar chains, etc., to the speedy analysis of trace chemical substances, and to high throughput screening of medical products and drugs, which are performed on a microchip with microchannels (extremely thin flow channels).
Such micro-sizing of a system has advantages including: (1) the consumption and waste of samples and reagents used in a chemical reaction and an antigen-antibody reaction can be reduced in quantity; (2) the power source necessary for a process can be reduced; (3) the increase in the ratio of surface area to volume can speed up heat transfer and substance transfer, and hence expectable results are: accurate control of reaction and separation; speeding up and promotion of high efficiency; and suppression of side reactions; (4) a multitude of samples can be handled on the same substrate; and (5) steps from sampling to detection can be performed on the same substrate. As a result, an actualization of a space-saving, portable, and inexpensive system can be conceived.
On the other hand, such micro-sizing of a system has disadvantages including: (1) detection sensitivity decreases in many cases due to a decrease in detection area; (2) it is difficult to generate a turbulent flow in a micro-scale fluid flow, and hence when reagents and the like is mixed, the mixture is dispersive, which takes time be well mixed; and (3) if bubbles or the like is generated, it is difficult to remove them due to an influence of surface tension, often leading to a tremendous influence on a measurement system.
With these advantages and disadvantages, microfluidic technology has been examined, and has been applied to: an acceleration sensor, a pressure sensor, a position sensor (gyroscope) and the like in the field of the automobile industry; a light guide, an optical switch, a minor, a lens, and the like in the field of telecommunication; and blood analysis, DNA analysis, chemical crime investigation, and the like in the field of life science. As a result, microfluidic technology has come to be found in our daily lives. In addition, it has been further applied to the fields of food, environmental testing, and ammunition.
The microfluidics technology currently under development is for application to sensors in many cases. Reported examples of such sensors include micro sensors which utilize: an immunoenzymatic reaction or antigen-antibody reaction; an ion-sensitive field effect (ISFET); a microelectrode; a microcantilever; acoustic waves; and resonance. Often reported examples of applications thereof include: a micro electrophoresis chip; a micro PCR (Polymerase Chain Reaction) chip; a micro gas chromatography chip; a micro liquid chromatography chip; and a DNA separation chip. Furthermore, the development of Lab-on-a-Chip in which the steps from sampling to analysis are performed on the same chip has been reported. Examples of this include: a multi-functional biochip using a nucleic acid or antibody specific to anthrax bacteria and coli bacilli; a portable measurement instrument for monitoring glucose, lactose, etc.; and laboratory test chip using an antigen-antibody reaction.
In fabricating a microfluidic device as described above, it is necessary to control the flow of fluids, for example to flow, stop, or one-directionally flow a plurality of micro fluids.
Non-Patent Document 1 describes a microfluidic device including: a liquid flow channel formed of silicone rubber; and a pressurization gap portion which is formed so as to be separated from the flow channel by a separation wall of silicone rubber. In addition, it describes an active fluid control method in which pressurized air is introduced into the pressurization gap portion to deflect the silicone rubber separation wall into the flow channel side, and with this deflection, the cross-sectional area of the flow channel is changed to thereby control the flow of a liquid.
However, this microfluidic device is made of a flexible material with low rigidity. Therefore, disadvantages arise such as it is difficult to manufacture a thin device because the material is low in pressure resistance and excessive in flexibility, and the device becomes complicated because it is an active device which requires a signal from the outside.
The micro valve described in FIG. 1 of Patent Document 1 includes: a thick, first pipe; a plurality of thin pipes which are formed thinner than the first pipe, one end of the thin pipes being connected so as to be in communication with the first pipe; and a second pipe which is formed thicker than the thin pipe and is connected so as to be in communication with the other end of the thin pipes, in which internal walls of the thin pipes are formed to be hydrophobic. According to this micro valve, when a liquid is introduced into the first pipe, the position of the liquid in the first pipe can be optionally controlled depending on a pressure difference between the pressure on the first pipe side and the pressure on the second pipe side across the liquid as a boundary. However, in the micro valve of Patent Document 1, it is very difficult to form a plurality of thin pipes. In addition, there is also a possibility that the thin pipe will be damaged if the pressure difference is too large. Moreover, a treatment to make these thin pipe portions specifically hydrophobic is also very difficult. Furthermore, the thin pipes of Patent Document 1 do not exert a function as a check valve, and cannot generate a pressure difference without using a pump.
The micro valve described in FIG. 3 of Patent Document 2 includes a valve mechanism formed of two micro flow channel chips made of polydimethylsiloxane (PDMS) and one membrane, in which the membrane undergoing displacement in a valve region is attached to and detached from a valve seat to thereby open and close a working fluid channel. Furthermore, in this micro valve, a drive fluid channel with a pressure chamber on which a pressure of a drive fluid acts in the valve region is formed by attachment to the membrane. It is configured such that the membrane is displaced by supply and exhaust of the pressure of the drive fluid to and from the pressure chamber to be attached to and detached from the valve seat, to thereby open and close as a one-way valve. However, in the micro valve described in Patent Document 2, the membrane which is attached to and detached from the valve seat is one-directionally displaced only toward the pressure chamber. Therefore, the gap between the membrane and the valve seat when the valve is opened is insufficient, and hence fluidity of the fluid is low. This is a cause of the occurrence of a pulsating flow. In addition, the pressure of the drive fluid is supplied from a vacuum pump via a glass pipe, making all of the equipment complicated and expensive.    Non-Patent Document: SCIENCE (Vol. 288, pp. 113-6 (2000))    Patent Document 1: Japanese Unexamined Patent Publication, First Publication No. 2000-27813    Patent Document 2: Japanese Patent No. 3418727